Energy storage for combustion turbine using molten carbonate electrolyzer cell

ABSTRACT

An energy storage system includes: a combustion turbine configured to output heated sweep gas; a reformer configured to receive natural gas and steam and to output reformed natural gas; a molten carbonate electrolyzer cell (“MCEC”) comprising an MCEC anode and an MCEC cathode, wherein the MCEC is configured to operate in a hydrogen-generation mode in which: the MCEC anode receives the reformed natural gas from the reformer, and outputs MCEC anode exhaust that contains hydrogen, and the MCEC cathode is configured to receive heated sweep gas from the combustion turbine, and to output MCEC cathode exhaust; and a storage tank configured to receive the MCEC anode exhaust that contains hydrogen.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of and priority to U.S.Provisional Application No. 62/806,995, filed Feb. 18, 2019, which ishereby incorporated by reference herein in its entirety.

BACKGROUND

The present application relates generally to the field of energy storageusing fuel cells.

Energy storage may be performed by generating H₂ (“hydrogen”) from wateror hydrocarbons. Storing energy using conventional water electrolyzerscan be inefficient and may require temperature control above thetemperature available from a combustion turbine, increasing the cost andenergy consumption for energy storage. Power levels on the order of 30to 60 kWh per kilogram of hydrogen produced can be required forconventional high temperature and room temperature electrolyzers.

SUMMARY

Systems and methods of the present disclosure relate to an energystorage system which can store excess energy as hydrogen and produceadditional hydrogen from methane. A molten carbonate electrolyzer cell(“MCEC”) (also called a reformer-electrolyzer-purifier (“REP”)) may beused to generate H₂. Examples of REPs and systems that include them aredescribed in PCT Publication No. WO 2015/116964, which is assigned tothe assignee of the present application. Power levels below 8 kWh perkilogram of hydrogen produced can be achieved with this technology whenpartially reformed natural gas feed and steam are fed to the MCEC.

One embodiment relates to an energy storage system which includes acombustion turbine configured to output heated sweep gas. The energystorage system further includes a reformer configured to receive naturalgas and steam and to output partially reformed natural gas. The energystorage system further includes a MCEC. The MCEC includes a MCEC anodeconfigured to receive the partially reformed natural gas from thereformer. The MCEC anode is configured to output MCEC anode exhaust thatcontains greater amount of hydrogen than the partially reformed naturalgas it receives from the reformer. The MCEC includes a MCEC cathodeconfigured to receive heated sweep gas from the combustion turbine. TheMCEC cathode is configured to output MCEC cathode exhaust. The MCEC isconfigured to operate in a hydrogen-generation mode. The energy storagesystem further includes a storage tank configured to receive the MCECanode exhaust that contains hydrogen.

In one aspect of the energy storage system, which is combinable with theabove embodiments and aspects in any combination, an electrochemicalhydrogen compressor (“EHC”) includes an EHC anode configured to receivethe MCEC anode exhaust. The EHC includes an EHC cathode configured tooutput a purified, pressurized hydrogen-containing stream to the storagetank.

In one aspect of the energy storage system, which is combinable with theabove embodiments and aspects in any combination, the EHC anode isconfigured to output an EHC anode exhaust that contains unrecoveredhydrogen and non-hydrogen fuel.

In one aspect of the energy storage system, which is combinable with theabove embodiments and aspects in any combination, a burner configured toreceive the MCEC cathode exhaust and the EHC anode exhaust, and toincrease a temperature of the reformer.

In one aspect of the energy storage system, which is combinable with theabove embodiments and aspects in any combination, a burner is configuredto receive the heated sweep gas from the combustion turbine and the EHCanode exhaust. The burner is configured to further heat the heated sweepgas before the heated sweep gas is received by the MCEC cathode.

In one aspect of the energy storage system, which is combinable with theabove embodiments and aspects in any combination, a methanation catalystis configured to receive the MCEC anode exhaust and to convert carbonmonoxide in the MCEC anode exhaust into methane.

In one aspect of the energy storage system, which is combinable with theabove embodiments and aspects in any combination, the combustion turbineis configured to receive part of the reformed natural gas from thereformer as would be the case when no excess power is available forstorage.

In one aspect of the energy storage system, which is combinable with theabove embodiments and aspects in any combination, a proton exchangemembrane (PEM) fuel cell is configured to receive the MCEC anode exhaustfrom the MCEC and/or a hydrogen-containing stream from the storage tank,and to output electricity.

In one aspect of the energy storage system, which is combinable with theabove embodiments and aspects in any combination, the PEM fuel cell isconfigured to output unreacted fuel. The burner is configured to receivethe MCEC cathode exhaust, the EHC anode exhaust (if an EHC is includedin the system), and the unreacted fuel from the PEM fuel cell, and toincrease a temperature of the reformer.

In one aspect of the energy storage system, which is combinable with theabove embodiments and aspects in any combination, a compressor isconfigured to pressurize and store hydrogen from the MCEC anode exhaust.

In one aspect of the energy storage system, which is combinable with theabove embodiments and aspects in any combination, a proton exchangemembrane (PEM) fuel cell is configured to receive a purifiedhydrogen-containing stream from the EHC cathode and/or the storage tank,and to output electricity. the MCEC is configured to operate in apower-generation mode in which the MCEC operates in reverse relative tothe hydrogen-generation mode.

In one aspect of the energy storage system, which is combinable with theabove embodiments and aspects in any combination, the reformer is areformer and heat recovery steam generator and steam from the heatrecovery steam generator is supplied to the reformer.

In one aspect of the energy storage system, which is combinable with theabove embodiments and aspects in any combination, the steam islow-pressure steam.

In one aspect of the energy storage system, which is combinable with theabove embodiments and aspects in any combination, the steam ismedium-pressure steam.

Another embodiment relates to an energy storage method utilizing amolten carbonate electrolyzer cell (“MCEC”) comprising an MCEC anode andan MCEC cathode, wherein the MCEC is configured to operate in ahydrogen-generation mode and the method comprises outputting heatedsweep gas from a combustion turbine; receiving natural gas and steam ata reformer and outputting reformed natural gas; receiving the reformednatural gas from the reformer at the MCEC anode and outputting MCECanode exhaust that contains hydrogen; receiving the heated sweep gasfrom the combustion turbine at the MCEC cathode and outputting MCECcathode exhaust; and receiving the MCEC anode exhaust that containshydrogen at a storage tank.

In one aspect of the energy storage method, which is combinable with theabove embodiments and aspects in any combination, the method furthercomprises receiving the MCEC anode exhaust at an EHC anode; andoutputting a purified hydrogen-containing stream from an EHC cathode tothe storage tank.

In one aspect of the energy storage method, which is combinable with theabove embodiments and aspects in any combination, the method furthercomprises receiving the MCEC anode exhaust at a methanation catalyst;and converting carbon monoxide in the MCEC anode exhaust into methane.

In one aspect of the energy storage method, which is combinable with theabove embodiments and aspects in any combination, the method furthercomprises receiving a purified hydrogen-containing stream from the EHCcathode and/or the storage tank; and outputting electricity.

In one aspect of the energy storage method, which is combinable with theabove embodiments and aspects in any combination, the method furthercomprises outputting excess fuel from the PEM fuel cell; receiving theMCEC cathode exhaust, EHC anode exhaust, and the excess fuel from thePEM fuel cell at a burner; and increasing a temperature of the reformer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an energy storage system, integrated witha combustion turbine combined cycle system, according to an exemplaryembodiment.

FIG. 2 is a schematic view of another embodiment of the energy storagesystem, which directs a portion of the reformed natural gas from thereformer to the combustion turbine, according to an exemplaryembodiment.

FIG. 3 is a schematic view of another embodiment of the energy storagesystem, which uses a compressor for hydrogen storage, according to anexemplary embodiment.

FIG. 4 is a schematic illustration of the prior art showing a combustionturbine combined cycle plant without energy storage.

DETAILED DESCRIPTION

A schematic illustration of a known combustion turbine combined cyclesystem is shown in FIG. 4. The system 400 includes a combustion turbine420 (e.g., gas turbine). Air 410 is supplied to the combustion turbine420 via the turbine compressor 416 to produce compressed air 418. Fuel414, such as natural gas, desulfurized natural gas, or other suitablefuel, is supplied to a combustion turbine burner 412. The compressed air418 is mixed with the fuel 414 and undergoes combustion, generating hightemperature gas 424 which is fed to the turbine expander 422 to generatepower. Heated gas 426 is output from the combustion turbine 420 and sentto the heat recovery steam generator (HRSG) 450. In the HRSG, boilerfeed water 442 is converted into high pressure steam 444. High pressuresteam 444 is output from the HRSG 450 and supplied to steam turbine 460.Steam 446 at a lower pressure than the high pressure steam 444 is outputfrom steam turbine 460 and supplied to steam turbine 470. In steamturbine 460 and steam turbine 470, the pressure of the high pressuresteam 444 is reduced as the steam turbine 460 and steam turbine 470generates power to increase the efficiency of the system 400. Steam 447at a lower pressure than steam 446 (e.g., below atmospheric pressure) isoutput from steam turbine 470 and supplied to a condenser 471, where thesteam is condensed under vacuum and the condensate can be pumped back tohigh pressure and used boiler feed water 442. Often, the steam isreheated between the steam turbines (not shown) to further increasepower output and overall system efficiency.

While this system 400 can be cost effective and efficient for base loadpower generation, the system 400 has little load following capabilitywithout loss in efficiency and has no capability to store excess powerfrom the grid. Therefore, the MCEC process described in the presentdisclosure can be added to a combustion turbine combined cycle system toprovide efficient energy storage and efficient peak power generation.

An energy storage system according to various exemplary embodiments willbe described below. The system includes a combustion turbine (e.g., gasturbine). Heated sweep gas generated by the gas turbine is used toreform natural gas and steam to output reformed natural gas. Thereformed natural gas is supplied to a molten carbonate electrolyzer cell(“MCEC”) (also called a reformer-electrolyzer-purifier (“REP”)), whichgenerates a hydrogen-containing stream. The hydrogen-containing streammay be used in conjunction with an electrochemical hydrogen compressor(“EHC”) to further purify and pressurize the hydrogen-containing stream.The hydrogen-containing stream can be stored in a storage tank for usein various applications, such as in a PEM fuel cell for peak powergeneration.

The energy storage system advantageously uses recovered heat from thecombustion turbine by directing the heated sweep gas to a reformer. Byproducing hydrogen from methane in addition to electrolysis hydrogen,less power is required as compared with conventional electrolysissystems. The power consumption is on the order of 8 kWh per kilogram ofhydrogen produced for a system without a EHC, and an additional 5 to 15kWh per kilogram of hydrogen produced for a system including a EHC, wellbelow typical systems that store excess power as hydrogen that use waterelectrolysis and require a power input of 45 to 60 kWh per kilogram ofhydrogen produced.

Referring to FIG. 1, an energy storage system 100 is shown according toan exemplary embodiment. The system 100 includes a combustion turbine120 (e.g., gas turbine). Air 110 is supplied to the combustion turbine120. Fuel 114, such as natural gas, desulfurized natural gas, or othersuitable fuel, is supplied to a combustion turbine burner 112. The air110 is mixed with the fuel 114 and undergoes combustion, generatingheated sweep gas 118. Heated sweep gas 118 is output from the combustionturbine 120.

The system 100 further includes a reformer and HRSG 150, which mayinclude a steam methane reformer, or other suitable hydrocarbonreformer. Boiler feed water 142 is supplied to the reformer and HRSG150. Steam 144 is output from the reformer and HRSG 150. The steam 144is supplied to a steam turbine 160 to generate additional power. Thesteam turbine 160 outputs low-pressure steam 146. Low-pressure steam 146can be steam at 15 psia. A portion of the low-pressure steam 148 ismixed with natural gas 152, such as desulfurized natural gas, or othersuitable fuel to create a low-pressure steam and natural gas mixture154. The low-pressure steam and natural gas mixture 154 is supplied tothe reformer and HRSG 150. Heated sweep gas 118 originating from thecombustion turbine 120 is used to reform the low-pressure steam andnatural gas mixture 154 to output reformed natural gas 122. A portion ofthe low-pressure steam 146 that is not mixed with the natural gas 152 issupplied to a second steam turbine 170. The steam turbine 170 outputsvery low-pressure steam 147 (e.g., less than atmospheric pressure) whichis condensed under vacuum by cooling in condenser 171.

The system 100 further includes a MCEC 130. The reformed natural gas 122is supplied to a MCEC 130 operating in a hydrogen-generation mode. TheMCEC 130 may be a MCEC assembly including a plurality of electrolyzerfuel cells formed in a fuel cell stack. The MCEC 130 includes a MCECanode 130A and a MCEC cathode 130B. The MCEC anode 130A receives thereformed natural gas 122 from the reformer and HRSG 150, and outputsMCEC anode exhaust 124 that contains hydrogen. The MCEC cathode 130Breceives heated sweep gas 118 from the combustion turbine 120, andoutputs MCEC cathode exhaust 126.

The heated sweep gas 118 from the combustion turbine 120 is introducedto the MCEC cathode 130B, which reduces the concentration of CO₂ and O₂in the MCEC cathode 130B. This process results in a lower voltage acrossthe MCEC 130 and lower power consumption. If CO₂ and O₂ is desired as abyproduct, the system 100 may also operate without a portion of theheated sweep gas 118. However, the use of heated sweep gas 118 helps tomaintain a uniform temperature in the MCEC cathode 130B, therebymaximizing the life of the MCEC 130. The MCEC cathode 130B outputs MCECcathode exhaust 126, which can be supplied to the reformer and HRSG 150.

MCEC anode exhaust 124 containing hydrogen is output from the MCEC anode130A. The MCEC anode exhaust 124 containing hydrogen may include 95-98%H₂. The MCEC anode exhaust 124 may also contain excess fuel. Forexample, the MCEC anode exhaust may include 2-5% CO₂, methane, and CO ona dry basis. The MCEC anode exhaust 124 may pass across an optionalmethanation catalyst 125. The methanation catalyst 125 may convert CO tomethane, thereby removing essentially all of the CO in the MCEC anodeexhaust 124, and output a methanation catalyst exhaust 127 making thegas suitable for use in an EHC 140 or PEM fuel cell 180. If methanationis not incorporated, CO must be removed by another purification processbefore it can be used in a PEM peak power generator. One alternatepurification system is a pressure swing adsorption system (PSA). Afterthe methanization catalyst, the MCEC anode exhaust is passed to an EHC140, such as a PEM fuel cell operating in electrolyzer mode, whichelectrochemically pumps the hydrogen to a high pressure suitable forstorage and purifies the methanation catalyst exhaust 127 (e.g., to99.99% H₂ or greater). The EHC 140 includes an EHC anode 140A configuredto receive the MCEC anode exhaust 124 or the methanation catalystexhaust 127, and to output EHC anode exhaust 129. The EHC anode exhaust129 may contain excess fuel, and may be supplied to a burner 182. Theburner 182 can be used to increase a temperature of the reformer andHRSG 150. The EHC 140 includes an EHC cathode 140B configured to outputa pressurized, purified hydrogen-containing stream 128.

The purified hydrogen-containing stream 128 can be stored or useddirectly in applications requiring hydrogen. For example, a portion 132of the hydrogen-containing stream 128 may be stored in a storage tank133, and a portion 134 of the hydrogen-containing stream may be used ina PEM fuel cell 180 configured to output electricity during peak powergeneration. The purified hydrogen-containing stream may be controllablystored in the storage tank 133 or sent immediately to the PEM fuel cell180, depending on current power demand.

Referring now to FIG. 2, an energy storage system 200 is shown accordingto a second exemplary embodiment. In energy storage system 200,partially reformed natural gas 213 is sent to the combustion turbine 220when excess power is not available rather than feeding the reformednatural gas 122 to the MCEC 130, as shown in FIG. 1. The partiallyreformed natural gas 213 provides an increase to the efficiency of thecombustion turbine 220. Further, the MCEC is heated by the heated sweepgas 218 and is ready to generate hydrogen and store power when needed.The system 200 includes a combustion turbine 220 (e.g., gas turbine).Air 210 is supplied to the combustion turbine 220. Fuel 214, such asnatural gas, desulfurized natural gas, or other suitable fuel, issupplied to a combustion turbine burner 212. The air 210 is mixed withthe fuel 214 and undergoes combustion, generating heated sweep gas 218.Heated sweep gas 218 is output from the combustion turbine 220.

The system 200 further includes a reformer and HRSG 250, which mayinclude a steam methane reformer, or other suitable hydrocarbonreformer. Boiler feed water 242 is supplied to the reformer and HRSG250. Steam 244 is output from the reformer and HRSG 250. The steam 244is supplied to a steam turbine 260. The steam turbine 260 outputsmedium-pressure steam 246 (e.g. 200 psia which is sufficiency high forthe reformed natural gas 213 to be fed to the pressurized combustionturbine burner 212). A portion of the medium-pressure steam 248 is mixedwith natural gas 252, such as desulfurized natural gas, or othersuitable fuel to create a medium-pressure steam and natural gas mixture254. The medium-pressure steam and natural gas mixture 254 is suppliedto the reformer and HRSG 250 which operates at a higher pressuresufficient to feed the combustion turbine 220. Partially reformed fuel213 supplied from the reformer and HRSG 250 to the combustion turbineburner 212 increases the amount of heat released by the fuel in theburner 212, thus requiring less fuel and increasing the efficiency. Theexcess steam in the reformer and HRSG 250 also increases the poweroutput when it is expanded in the combustion turbine expander 221.Heated sweep gas 218 originating from the combustion turbine 220 is usedto provide heat for reforming the medium-pressure steam and natural gasmixture 254 to output reformed natural gas 222. A portion of themedium-pressure steam 246 that is not mixed with the natural gas 252 issupplied to a steam turbine 270. The steam turbine 270 outputsadditional power and produces a low-pressure steam 247. The low-pressuresteam 247 is sent to a condenser 271 and the condensate is recycled tothe system 200 through a pump as boiler feed water 242. The reformednatural gas 213 can be all of the partially reformed natural gas, or anyportion of the reformed natural gas 222.

The system 200 further includes a MCEC 230. The reformed natural gas 222is supplied to a MCEC 230 operating in a hydrogen-generation mode. TheMCEC 230 can be a MCEC assembly including a plurality of electrolyzerfuel cells formed in a fuel cell stack. The MCEC 230 includes a MCECanode 230A and a MCEC cathode 230B. The MCEC anode 230A receives thepartially reformed natural gas 222 from the reformer and HRSG 250, andoutputs MCEC anode exhaust 224 that contains hydrogen. The MCEC cathode230B receives heated sweep gas 218 from the combustion turbine 220, andoutputs MCEC cathode exhaust 226.

The heated sweep gas 218 from the combustion turbine 220 can beintroduced to the MCEC cathode 230B, which reduces the concentration ofCO₂ and O₂ in the MCEC cathode 230B. This process results in a lowervoltage across the MCEC 230 and lower power consumption. If CO₂ and O₂is desired as a byproduct, the system 200 may also operate without aportion of the heated sweep gas 218. However, the use of heated sweepgas 218 helps to maintain a uniform temperature in the MCEC cathode230B, thereby maximizing the life of the MCEC 230. The MCEC cathode 230Boutputs MCEC cathode exhaust 226, which can be supplied to the reformerand HRSG 250.

MCEC anode exhaust 224 containing hydrogen is output from the MCEC anode230A. The MCEC anode exhaust 224 containing hydrogen can include 95-98%H₂. The MCEC anode exhaust 224 may also contain excess fuel. Forexample, the MCEC anode exhaust 224 may include 2-5% CO₂, methane, andCO on a dry basis. The MCEC anode exhaust 224 may pass across anoptional methanation catalyst 225. The methanation catalyst 225 mayconvert CO to methane, thereby removing essentially all of the CO in theMCEC anode exhaust 224, and output a methanation catalyst exhaust 227,making the gas suitable for use in an EHC 240 or PEM fuel cell 280. Themethanization catalyst produces a CO free MCEC anode exhaust which ispassed to an EHC 240, such as a PEM fuel cell operating in electrolyzermode, which electrochemically pumps the hydrogen to a high pressure andpurifies the methanation catalyst exhaust 227 (e.g., to 99.99% H₂ orgreater) in one step. The EHC 240 includes an EHC anode 240A configuredto receive the MCEC anode exhaust 224 or the methanation catalystexhaust 227, and to output EHC anode exhaust 229. The EHC anode exhaust229 may contain excess fuel, and may be supplied to a burner 282. Theburner 282 can be configured to receive the heated sweep gas 218 fromthe combustion turbine 220 and the EHC anode exhaust 229 and to furtherheat the heated sweep gas 218 before the heated sweep gas 218 isreceived by the MCEC cathode 230B. The burner 282 can be used toincrease a temperature of the MCEC 230, instead of the reformer and HRSG150 as shown in FIG. 1. The EHC 240 includes an EHC cathode 240Bconfigured to output a pressurized, purified hydrogen-containing stream228.

The purified hydrogen-containing stream 228 can be stored or useddirectly in applications requiring hydrogen. For example, a portion 232of the hydrogen-containing stream 228 may be stored in a storage tank233, and a portion 234 of the hydrogen-containing stream may be used ina PEM fuel cell 280 configured to output electricity during peak powergeneration. The purified hydrogen-containing stream may be controllablystored in the storage tank 233 or sent immediately to the PEM fuel cell280, depending on current power demand.

Referring now to FIG. 3, an energy storage system 300 is shown accordingto a third exemplary embodiment. In energy storage system 300, acompressor 340, rather than an EHC, is used prior to hydrogen storage.In this configuration, a hydrogen-containing stream from an MCEC anode(e.g. 95-98% H₂) is sent to the storage tank 333 and/or PEM 380. Thesystem 300 includes a combustion turbine 320 (e.g., gas turbine). Air310 is supplied to the combustion turbine 320. Fuel 314, such as naturalgas, desulfurized natural gas, or other suitable fuel, is supplied to acombustion turbine burner 312. The air 310 is mixed with the fuel 314and undergoes combustion, generating heated sweep gas 318. Heated sweepgas 318 is output from the combustion turbine 320.

The system 300 further includes a reformer and HRSG 350, which mayinclude a steam methane reformer, or other suitable hydrocarbonreformer. Boiler feed water 342 is supplied to the reformer and HRSG350. Steam 344 is output from the reformer and HRSG 350. The steam 344is supplied to a steam turbine 360. The steam turbine 360 outputslow-pressure steam 346. Low-pressure steam 346 can be steam at 15 psia.A portion of the low-pressure steam 348 is mixed with natural gas 352,such as desulfurized natural gas, or other suitable fuel to create alow-pressure steam and natural gas mixture 354. The low-pressure steamand natural gas mixture 354 is supplied to the reformer and HRSG 350.Heated sweep gas 318 originating from the combustion turbine 320 is usedto provide the heat needed to reform the low-pressure steam and naturalgas mixture 354 to output reformed natural gas 322. A portion of thelow-pressure steam 346 that is not mixed with the natural gas 352 issupplied to a second steam turbine 370. The steam turbine 370 outputsvery low-pressure steam 347 (e.g, less than atmospheric pressure) whichis condensed under vacuum by cooling in condenser 371.

The system 300 further includes a MCEC 330. The reformed natural gas 322is supplied to a MCEC 330 operating in a hydrogen-generation mode. TheMCEC 330 may be a MCEC assembly including a plurality of electrolyzerfuel cells formed in a fuel cell stack. The MCEC 330 includes a MCECanode 330A and a MCEC cathode 330B. The MCEC anode 330A receives thereformed natural gas 322 from the reformer and HRSG 350, and outputsMCEC anode exhaust 324 that contains hydrogen. The MCEC cathode 330Breceives heated sweep gas 318 from the combustion turbine 320, andoutputs MCEC cathode exhaust 326.

The heated sweep gas 318 from the combustion turbine 320 is introducedto the MCEC cathode 330B, which reduces the concentration of CO₂ and O₂in the MCEC cathode 330B. This process results in a lower voltage acrossthe MCEC 330 and lower power consumption. If CO₂ and O₂ is desired as abyproduct, the system 300 may also operate without a portion of theheated sweep gas 318. However, the use of heated sweep gas 318 helps tomaintain a uniform temperature in the MCEC cathode 330B, therebymaximizing the life of the MCEC 330. The MCEC cathode 330B outputs MCECcathode exhaust 326, which can be supplied to the reformer and HRSG 350.

MCEC anode exhaust 324 containing hydrogen is output from the MCEC anode330A. The MCEC anode exhaust 324 can include 95-98% H₂. The MCEC anodeexhaust 324 may also contain excess fuel. For example, the MCEC anodeexhaust 324 can include 2-5% CO₂, methane, and CO on a dry basis. TheMCEC anode exhaust 324 may pass across an optional methanation catalyst325A. The methanation catalyst 325A may convert CO to methane, therebyremoving essentially all of the CO in the MCEC anode exhaust 324, andoutput a methanation catalyst exhaust 327 making the gas suitable foruse in a PEM fuel cell 380. The MCEC anode exhaust 324 may be methanatedas the MCEC anode exhaust 234 is cooled to remove essentially all CO andmake the MCEC anode exhaust 324 suitable for use as fuel in a PEM powergenerator. Alternately, the CO may be removed by a PSA 325B downstreamof the exhaust gas compressor 340.

The MCEC anode exhaust 324 may be supplied to a compressor 340. Thecompressor 340 outputs a pressurized hydrogen-containing stream 328. Thehydrogen-containing stream 328 can be stored or used directly inapplications requiring hydrogen. For example, a portion 332 of thehydrogen-containing stream 328 may be stored in a storage tank 333, anda portion 334 of the hydrogen-containing stream may be used in a PEMfuel cell 380 configured to output electricity during peak powergeneration. The hydrogen-containing stream may be controllably stored inthe storage tank 333 or sent immediately to the PEM fuel cell 380,depending on current power demand. Generally during peak power demand,the MCEC 330, which uses power, will be turned off and the PEM fuel cellwill be fed from the storage tank 333 in order to maximize the net powergenerated. In some cases, the MCEC 330 will be operated in a manner toproduce power to further increase the peak power generation.

While the purity of hydrogen received in the storage tank 333 and/or PEMfuel cell 380 is lower than in the first two embodiments describedabove, the elimination of the EHC offers a lower cost option. Becausethe hydrogen stored in the storage tank 333 is approximately 95-98% H₂,the PEM fuel cell 380 can output excess fuel 335 (e.g., hydrogen fueland non-hydrogen fuel). The excess fuel 335 can be supplied to a burner382, which can be used to heat the reformer and HRSG 350.

According to certain embodiments, the MCEC 130, 230, 330 can operatingin a power-generation mode in which the MCEC 130, 230, 330 operates inreverse relative to the hydrogen-generation mode. The operation of theMCEC 130, 230, 330 in reverse allows the MCEC 130, 230, 330 to operateas a conventional fuel cell to receive hydrogen as fuel and generatepower.

As utilized herein, the terms “approximately,” “about,” “substantially,”and similar terms are intended to have a broad meaning in harmony withthe common and accepted usage by those of ordinary skill in the art towhich the subject matter of this disclosure pertains. It should beunderstood by those of skill in the art who review this disclosure thatthese terms are intended to allow a description of certain featuresdescribed and claimed without restricting the scope of these features tothe precise numerical ranges provided. Accordingly, these terms shouldbe interpreted as indicating that insubstantial or inconsequentialmodifications or alterations of the subject matter described and claimedare considered to be within the scope of this disclosure as recited inthe appended claims.

It should be noted that the term “exemplary” as used herein to describevarious embodiments is intended to indicate that such embodiments arepossible examples, representations, and/or illustrations of possibleembodiments (and such term is not intended to connote that suchembodiments are necessarily extraordinary or superlative examples).

The terms “coupled,” “connected,” and the like as used herein mean thejoining of two members directly or indirectly to one another. Suchjoining may be stationary (e.g., permanent) or moveable (e.g., removableor releasable). Such joining may be achieved with the two members or thetwo members and any additional intermediate members being integrallyformed as a single unitary body with one another or with the two membersor the two members and any additional intermediate members beingattached to one another.

References herein to the position of elements (e.g., “top,” “bottom,”“above,” “below,” etc.) are merely used to describe the orientation ofvarious elements in the FIGURES. It should be noted that the orientationof various elements may differ according to other exemplary embodiments,and that such variations are intended to be encompassed by the presentdisclosure.

It is to be understood that although the present invention has beendescribed with regard to preferred embodiments thereof, various otherembodiments and variants may occur to those skilled in the art, whichare within the scope and spirit of the invention, and such otherembodiments and variants are intended to be covered by correspondingclaims. Those skilled in the art will readily appreciate that manymodifications are possible (e.g., variations in sizes, structures,values of parameters, mounting arrangements, use of materials,orientations, manufacturing processes, etc.) without materiallydeparting from the novel teachings and advantages of the subject matterdescribed herein. For example, the order or sequence of any process ormethod steps may be varied or re-sequenced according to alternativeembodiments. Other substitutions, modifications, changes and omissionsmay also be made in the design, operating conditions and arrangement ofthe various exemplary embodiments without departing from the scope ofthe present disclosure.

What is claimed is:
 1. An energy storage system comprising: a combustionturbine configured to output heated sweep gas; a reformer configured toreceive natural gas and steam and to output reformed natural gas; amolten carbonate electrolyzer cell (“MCEC”) comprising an MCEC anode andan MCEC cathode, wherein the MCEC is configured to operate in ahydrogen-generation mode in which: the MCEC anode is configured toreceive the reformed natural gas from the reformer, and outputs MCECanode exhaust that contains hydrogen, and the MCEC cathode is configuredto receive heated sweep gas from the combustion turbine, and to outputMCEC cathode exhaust; and a storage tank configured to receive the MCECanode exhaust that contains hydrogen.
 2. The energy storage system ofclaim 1, further comprising: an electrochemical hydrogen compressor(“EHC”) comprising: an EHC anode configured to receive the MCEC anodeexhaust, and an EHC cathode configured to output a purifiedhydrogen-containing stream to the storage tank.
 3. The energy storagesystem of claim 2, wherein the EHC anode is configured to output an EHCanode exhaust that contains excess fuel.
 4. The energy storage system ofclaim 3, further comprising: a burner configured to receive the MCECcathode exhaust and the EHC anode exhaust, and to increase a temperatureof the reformer.
 5. The energy storage system of claim 3, furthercomprising: a burner configured to receive the heated sweep gas from thecombustion turbine and the EHC anode exhaust, and to further heat theheated sweep gas before the heated sweep gas is received by the MCECcathode.
 6. The energy storage system of claim 2, further comprising: amethanation catalyst configured to receive the MCEC anode exhaust and toconvert carbon monoxide in the MCEC anode exhaust into methane.
 7. Theenergy storage system of claim 2, a proton exchange membrane (PEM) fuelcell configured to receive a purified hydrogen-containing stream fromthe EHC cathode and/or the storage tank, and to output electricity. 8.The energy storage system of claim 1, further comprising: a protonexchange membrane (PEM) fuel cell configured to receive the MCEC anodeexhaust from the MCEC and/or a hydrogen-containing stream from thestorage tank, and to output electricity.
 9. The energy storage system ofclaim 8, wherein: the PEM fuel cell is configured to output excess fuel,and the energy storage system further comprises a burner configured toreceive the MCEC cathode exhaust, EHC anode exhaust, and the excess fuelfrom the PEM fuel cell, and to increase a temperature of the reformer.10. The energy storage system of claim 1, further comprising: acompressor configured to receive and pressurize hydrogen from the MCECanode exhaust.
 11. The energy storage system of claim 1, wherein thecombustion turbine is configured to receive part of the reformed naturalgas from the reformer.
 12. The energy storage system of claim 1, whereinthe MCEC is configured to operate in a power-generation mode in whichthe MCEC operates in reverse relative to the hydrogen-generation mode.13. The energy storage system of claim 1, further comprising: a heatrecovery steam generator configured to generate the steam and supply thesteam to the reformer.
 14. The energy storage system of claim 13,wherein the heat recovery steam generator is configured to generatelow-pressure steam.
 15. The energy storage system of claim 13, whereinthe heat recovery steam generator is configured to generatemedium-pressure steam.
 16. An energy storage method utilizing a moltencarbonate electrolyzer cell (“MCEC”) comprising an MCEC anode and anMCEC cathode, wherein the MCEC is configured to operate in ahydrogen-generation mode and the method comprises: outputting heatedsweep gas from a combustion turbine; receiving natural gas and steam ata reformer and outputting reformed natural gas from the reformer;receiving the reformed natural gas from the reformer at the MCEC anodeand outputting MCEC anode exhaust that contains hydrogen from the MCECanode; receiving the heated sweep gas from the combustion turbine at theMCEC cathode and outputting MCEC cathode exhaust from the MCEC cathode;and receiving the MCEC anode exhaust that contains hydrogen at a storagetank.
 17. The energy storage method of claim 16, further comprising anelectrochemical hydrogen compressor (“EHC”), the method comprising:receiving the MCEC anode exhaust at an EHC anode; and outputting apurified hydrogen-containing stream from an EHC cathode to the storagetank.
 18. The energy storage method of claim 17, comprising: receivingthe MCEC anode exhaust at a methanation catalyst; and converting carbonmonoxide in the MCEC anode exhaust into methane.
 19. The energy storagemethod of claim 17, receiving a purified hydrogen-containing stream fromthe EHC cathode and/or the storage tank; and outputting electricity. 20.The energy storage method of claim 17, outputting excess fuel from aproton exchange membrane (PEM) fuel cell; receiving the MCEC cathodeexhaust, EHC anode exhaust, and the excess fuel from the PEM fuel cellat a burner, and increasing a temperature of the reformer using theburner.